1. Field of the Invention
The present invention relates to an image display device and a driving method for the same, and more specifically to the image display device and the driving method employed in the same that are well suited for applications, for example, in a case where a display having a memory property such as electronic paper is used.
2. Description of the Related Art
Recently, a content display device replacing paper has been needed socially as a medium for displaying content. This type of content display device may come in a browser terminal having an A5 (148 mm×210 mm) size to A4 (210 mm×297 mm) size display screen capable of browsing news updated as needed, for example, the most recent newspaper contents delivered automatically from a server or a large-scale advertizing billboard having a display screen of an A2 (420 mm×594 mm) size to A1 (594 mm×841 mm) size or larger which may be put up at a station. The content display devices give and receive contents in condition where they are wirelessly connected with the server and also are needed to operate on low dissipation power with less loads on environments in a condition where it is supplied with power from a secondary battery or a solar battery.
In such a case, the content display device should preferably be made up of, for example, a high-definition display with a relatively large screen, which has a memory property, a radio transmit/receive unit, and a low-dissipation power display circuit such as a control unit having a smaller number of components. For example, a book viewer Kindle (registered trademark) from Amazon Inc. has A6 (105 mm×148 mm) size electronic paper with a resolution of a SVGA (super video graphics array; 800 by 600 matrix of pixels) using micro-capsule type electrophoretic display elements from the American E Ink Company and so has a function to wirelessly connect to a contents server via the internet so that books, blogs, newspapers, and magazines can be directly downloaded online not by way of a personal computer.
FIG. 14 is an outlined cross-sectional view for showing the configuration of the micro-capsule type electrophoretic display element used in the related-art type of content display device.
As shown in FIG. 14, the electrophoretic display element is made of a stack including a thin film transistor (TFT) glass substrate 1, an electrophoretic element film 2, and a facing substrate 3. The TFT glass substrate 1 has a switching element TFT 4 and a pixel electrode 5 connected to the TFT 4. The electrophoretic element film 2 has a polymer binder filled with micro-capsules 6 having an about 40 μm size. The micro-capsule 6 has a solvent injected therein, in which solvent negatively-charged white pigments and positively-charged black pigments having a scale of nanometers are suspended in countless numbers. The facing substrate 3 has a facing electrode 7 formed thereon for giving a reference potential.
In the display device using this type of electrophoretic display element, a voltage that corresponds to image data is applied between the pixel electrode 5 and the facing electrode 7, causing the white pigments and the black pigments to move up and down. For example, supposing the side of the facing electrode 7 to be a display surface, if the pixel electrode 5 is supplied with a positive voltage, the negatively-charged white pigments get close to the pixel electrode 5 so that the display surface may appear black, while on the other hand, if the pixel electrode 5 is supplied with a negative voltage, the positively-charged black pigments get close to the pixel electrode 5 so that the display surface may appear white. Further, since the electrophoretic display elements have memory functions, when switching the pixel data of an image from white to black, a negative voltage is applied, whereas when switching the pixel data of the image from black to white, a positive voltage is applied; and when displaying white after white or black after black, a voltage of 0V is applied. That is, in driving this display device, the voltage of a signal which is applied to the electrophoretic display element is determined on the basis of comparison between the preceding screen and the next screen.
Further, typically, an active matrix display device such as a liquid crystal display (LCD) employs a lapse of time of 1/60 second (=16.6 ms) as one frame so that an entirety of an image may be switched in the one frame. On the other hand, a display device using an electrophoretic display element cannot switch the screen unless a voltage is applied over a period of a plurality of the frames because the electrophoretic display element has a low response speed and, therefore, employs a driving method of pulse width modulation (PWM), by which a constant voltage will be applied in a period of a plurality of the frames. In the case of the present display device in which micro-capsule type electrophoretic display elements are used, since such electrophoretic display elements have a memory property, when updating the screen, a history of the preceding screen needs to be erased. Therefore, either one of two driving methods is employed: one driving method (hereinafter referred to as “reset driving method”) of displaying an updating screen after erasing the entire screen by switching this to white, black, and white in this order on a reset screen or the other driving method (hereinafter referred to as “preceding screen referencing/driving method”) of determining a voltage to be applied to pixels by referencing a look-up table (LUT) based on pixel data of the preceding screen and that of the next screen. The preceding screen referencing/driving method need not use a reset screen and so is excellent in display performance but requires a graphic memory for storing the preceding screen and the updating screen and so has a problem in that a residual image of the preceding screen will appear unless the LUT is set properly. Further, it has another problem of an increased scale of the graphic memory and peripheral circuitry as well as increased dissipation power and a complicated hardware architecture.
Besides the driving methods for the above-mentioned display devices, this type of related art may include a driving method for a bistable electro-optic display described in Japanese Patent Application Publication No. 2007-249230 (hereinafter, referred to as Related Art Patent Document 1), for example.
In driving, this display stores data of a plurality of images in a data storage unit and also stores a preceding screen and an updating screen in a graphic memory made up of a static random access memory (SRAM) so that those two screens may be compared to each other. Therefore, the graphic memory needs to have a capacity for storing at least two screens: the updating screen and the preceding screen. The capacity of this graphic memory is of no problem in the case of a relatively small display size; however, in the case of large-size display, for example, monochromatic A4-size display in an ultra extended graphics array (UXGA) with 1600 by 1200 (pixels), this graphic memory needs to have a capacity of 30.8 Mbits (=1600×1200×8×2 (screens)), assuming that one pixel requires eight bits of data. Further, in the case of A2-size advertizing display in a quad ultra extended graphics array (QUXGA) with 3200 by 2400 (pixels), this graphic memory needs to have a capacity of 123.3 Mbits (=3200×2400×8×2 (screens)).
Further, in an electrophoretic display having compressed memory data described in Japanese Patent Application Publication No. 2007-510944 (hereinafter, referred to as Related Art Patent Document 2), the data of screens to be input to the graphic memory is compressed and stored in it, so that at the time of comparison for updating of the next screen, the compressed image data of a preceding screen is expanded in real time to create data to be compared to an image data stream of the next screen, thereby calculating a signal to be applied to the electrophoretic display based on a LUT. In this case, during a lapse of time when the image is held, the data of the preceding screen is compressed and saved, so that while this image is being held, it is necessary only to supply power to a memory that holds the data, thereby reducing dissipation power during image holding.
Further, in a display device described in Japanese Patent Application Publication No. 2005-242081 (hereinafter, referred to as Related Art Patent Document 3), a buffer for graphics will be dynamically allocated in a memory region, thereby effectively utilizing the capacity of a memory including the graphic memory. That is, if expansion/transfer of a display image starts, a memory region for expanding one screen ready for display is acquired from the RAM, so that when an object for the one screen is expanded, a display panel unit is supplied with power so that the display image may be updated. When the display image is updated completely, power supply to the display unit is cut off, thereby reducing dissipation power and also releasing the expansion memory.
It is considered that this example has employed the reset driving method of displaying an updating screen after erasing the history of the preceding screen from the display on a reset screen by switching the screen to white, black, and white in this order so that the residual image of the preceding screen may not be left, because the memory region for graphics is dynamically acquired and so the data of the preceding screen is not held. Therefore, a display controller is simplified.
Further, in an image display system described in Japanese Patent Application Publication No. 2001-166761 (hereinafter, referred to as Related Art Patent Document 4), if the side of a host that executes an application requests the side of a panel to display an image, data of the image before this image is expanded is transferred to the panel side. The panel side, which is equipped with a panel memory for image expansion, expands the image in the panel memory based on the image data transferred from the host side and also displays the image expanded into this panel memory in the panel.
Further, in an image display device described in Japanese Patent Application Publication No. 2007-010970 (hereinafter, referred to as Related Art Patent Document 5), an image is displayed on a display device including liquid crystal having a memory property; if a command is given to change an image being displayed on this display, a central processing unit (CPU) determines an update region in which at least part of the image on the display is to be updated, based on the currently displayed image and a post-change image. Then, a graphics processing unit (GPU) rewrites the display only in the region determined as the update region by the CPU.
However, the above-mentioned related arts have the following problems. That is, the bistable electro-optic display described in the Related Art Patent Document 1 has a problem in that since it is necessary to supply the graphic memory with data of two screens, that is, the preceding screen and the updating screen when updating an image, this graphic memory may be bloated to increase dissipation power and manufacturing costs. Further, although it is unnecessary to supply power to display unit having a memory property when an image is being held, since image data of the preceding screen is stored in the graphic memory when updating this image, in order to continue to hold a history of the preceding screen, it is necessary to continue supplying power to the graphic memory even during image holding other than image updating, so that a driving circuit dissipates power even during image holding, making it difficult to reduce dissipation power.
Further, the electrophoretic display described in the Related Art Patent Document 2 alleviates the problem of the Related Art Patent Document 1, that is, power will be dissipated by the driving circuit during image holding; however, when updating an image, it is necessary to expand data of the preceding screen and also add a memory and a circuit for holding the data of the updating screen expanded. Accordingly, the graphic memory itself is not downsized, making it difficult to reduce dissipation power, in particular, at the time of image updating.
In the case of the display device described in the Related Art Patent Document 3, although the display controller is simplified, it is necessary to insert a reset screen (for switching display to white, black, and white in this order) between a point in time for displaying the preceding screen and a point in time for displaying an updating screen; therefore, when displaying character data pieces continually, a problem occurs in response speed lowering and screen flickering, so that this type of display device is not suitable as a content display device. Further, for example, it is necessary to acquire memory dynamically, which leads to the necessity of an OS function such as memory management and also the necessity of securing a storage large enough to accommodate the memory dynamically, giving rise to a problem in an increase in manufacturing cost. Further, when updating an image, a graphic memory region having a size of one frame of image data is acquired, giving rise to a problem in that the necessary memory size or dissipation power will not be reduced.
The image display system described in the Related Art Patent Document 4 employs a concept different from that of the present invention, although processing pieces are dispersed between the host side and the panel side so that workloads of an entirety of the system may be optimized.
The image display device described in the Related Art Patent Document 5 employs a concept different from that of the present invention, although an image will be rewritten more speedily on a display including liquid crystal with memory without damaging a constant image quality.